[MINOR] Introduce a method for acquiring the number of data in local …

…Tablets (#173)

src/main/java/edu/snu/cay/services/et/evaluator/api/Tablet.java

@@ -106,4 +106,9 @@ public interface Tablet<K, V, U> {
    * @return an iterator of {@link Entry} of local data key-value pairs
    */
   Iterator<Entry<K, V>> getDataIterator();
+
+  /**
+   * @return the number of local data items existing in the Tablet.
+   */
+  int getNumDataItems();
 }

src/main/java/edu/snu/cay/services/et/evaluator/impl/TabletImpl.java

@@ -33,6 +33,7 @@
 @EvaluatorSide
 @ThreadSafe
 public final class TabletImpl<K, V, U> implements Tablet<K, V, U> {
+
   /**
    * Local blocks which this executor owns.
    */
@@ -158,4 +159,23 @@ public Entry<K, V> next() {
       }
     };
   }
+
+  @Override
+  public int getNumDataItems() {
+    int numTotal = 0;
+
+    // will not care blocks migrated in after this call
+    final List<Integer> localBlockIds = ownershipCache.getCurrentLocalBlockIds();
+
+    for (final Integer blockId : localBlockIds) {
+      try {
+        final Block<K, V, U> block = blockStore.get(blockId);
+        numTotal += block.getNumPairs();
+      } catch (final BlockNotExistsException e) {
+        // This block has moved out to another executor, but it is safe to ignore it in counting the number of items.
+      }
+    }
+
+    return numTotal;
+  }
 }